The use of ethyl silicate as a foundry or refractory binder becomes less attractive with time, on the grounds of cost and the increasing difficulty of meeting the health and safety requirements imposed upon its use.
A candidate for an alternative is a silica sol which is water based as opposed to alcohol based. Silica sol is a colloidal suspension, typically slightly alkali, p.H. c. 7.8, of amorphous silica particles in water. By various expedients known in the refractory art it can be converted to a hydrated silica gel which can be induced to set into a green shape. Indeed silica sol is used extensively in the foundry industry particularly in lost wax shell processes of precision casting. However, this relies upon a removal and drying out of water in order to achieve a green strength. This takes some time and is not amenable to proprietary processes such as the "Shaw Process" and the "Zirconal Process" which call for a fast controllable setting time of a "green" molding or refractory. Silica sols can indeed set under certain conditions but the green strength is low until some drying has taken place.
Phosphates are well recognised also as good refractory binders. The best known being acidic such as aluminium phosphate or indeed phosphoric acid. This acidity imposes limitations on their use especially in precision pieces or basic refractories. The use of neutral ammonium phosphates has been proposed, and I have a European patent on this, EP-B-0501662. The chemical sequence in the patent is:
Polyammonium phosphate+reactive basic magnesium refractory.fwdarw.magnesium phosphate gel+NH.sub.3, and then on heating magnesium phosphate+alumina.fwdarw.aluminium/magnesium spinel+"P.sub.2 O.sub.5 ".
By "P.sub.2 O.sub.5 " here is meant phosphate generally, and not necessarily phosphorus pentoxide. The stoichiometric content of materials disclosed herein is calculated on the basis of units of "P.sub.2 O.sub.5 " for simplicity.